A spark plug is used as ignition means of an internal combustion engine such as an automotive engine. In the spark plug, spark discharge is formed between a center electrode and a ground electrode. The center electrode is held in a shaft hole of the insulator. The ground electrode is fixed on a mounting bracket. The spark discharge is generated by applying a high voltage to the center electrode via a terminal fitting connected to a high-voltage source. In addition, a resistor is disposed between the center electrode and the terminal fitting for reducing radio noise. The radio noise is caused due to application of the high voltage.
Japanese Unexamined Patent Application Publication No. 2006-66086 will be referred to as patent document 1. In one instance, the followings are disclosed in patent document 1. In a shaft hole of the insulator, a resistor is disposed on a base end side of a center electrode. A distance between a tip end of the center electrode and a tip end of the resistor is set in a range of 10 mm to 20 mm. A diameter of the resistor is set in a range of 2.0 mm to 3.5 mm. In this way, a configuration satisfying both reduction of the radio noise and improvement of heat resistance is disclosed. In addition, an axial direction length of the resistor is 13 mm to 15 mm. Thereby, durability of the resistor is kept and noise suppressive performance (hereinafter referred to as a noise reduction property) is further improved. It is proposed that an axial direction length of a glass seal which contacts with the resistor is not more than 2 mm.
Recently, a discharge voltage generated in a spark discharge gap has tended to be increased. However, when the discharge voltage becomes high, a heat value of the resistor is increased. Thereby, oxidation of a conducting material making up the resistor is progressed, and degradation of the resistor tends to progress. Therefore, an increase in performance of the resistor at a higher required voltage is required.
In general, the length of the resistor needs to increase for improving the noise reduction property. However, when the length of the resistor is increased, a difficulty is found in that a density of the resistor becomes high during filling the resistor in the shaft hole. This leads to reduction of a load life. Therefore, as described in patent document 1, the axial direction length of the resistor is limited to be not more than 15 mm. In addition, the distance between the center electrode and the resistor and the axial direction length of the glass seal and the like are pre-determined. The center electrode is held in the shaft hole of the insulator. In this way, the density of the resistor needs to be suppressed from being reduced.